1. Field of the Invention
This invention relates generally to the global positioning system (GPS) and specifically to methods and devices that allow a user to reduce GPS positioning errors.
2. Description of Related Art
The global positioning system (GPS) is a United States funded satellite system consisting of sixteen to twenty-four satellites in a constellation that beams highly accurate, timed signals to earth. GPS receivers can process these signals from anywhere in the world and provide a user with position information that can generally be accurate to within 20 or 30 meters. Each GPS satellite moves through a known orbit, and the time that individual GPS satellite transmissions take to reach a user""s position are used in solving a triangulation problem. The individual signal paths from each satellite to the user""s position must pass through the troposphere and ionosphere. GPS errors can occur when radio signal propagation delays caused by the troposphere and ionosphere are not accurately accounted for. The ionosphere introduces its maximum delays at about 2:00 P.M. local time and has practically no effect at night. Thus, the local time may be important to GPS accuracy. The troposphere can introduce delays that are a function of barometric pressure, temperature, humidity and other weather variables. Variations in the actual orbit of each GPS satellite can also have an impact on GPS accuracy. Drifts in the clock of each GPS satellite will also impact user GPS accuracy.
GPS signals to positioning receivers that are close to each other on the earth""s surface are subjected to nearly identical signal propagation delays, satellite clock drift, and other sources of error. Because of this, if the position of one receiver (i.e., a reference receiver) is accurately known, it can provide data, (or data may be calculated remotely from the reference receiver) known as differential data or correction data, to nearby receivers known as rovers to compensate for errors that are common to receivers in close proximity to each other. This technique is known as differential GPS (DGPS), and it can be used to produce extremely accurate positional data for surveying, structural monitoring, etc. Service bureaus and agencies have been established to sell or otherwise provide differential data. Such data range from real-time local information to sometimes very complex mathematical models based on long-term observations. U.S. Government agencies issue orbit correction information on a satellite-by-satellite basis.
Dual frequency carrier GPS receivers continuously track P-code L1 and L2 carriers of a GPS satellite to generate accumulated delta-range measurements (ADR) and at the same time track L1 C/A-code to generate code-phase measurements. Each carrier is modulated with codes that leave the GPS satellite at the same clock time. Since the ionosphere produces different delays for radio carriers that have different radio frequencies, dual carrier receivers can be used to obtain real-time measurements of ionospheric delays at a user""s particular position. (L1 is typically 1575.42 MHz and L2 is typically 1227.6 MHz.). The L1 and L2 ADR measurements are combined to generate a new L1 ADR measurement that has an ionospheric delay of the same sign as the ionospheric delay in the L1 pseudorange.
Accurate ionospheric delay figures, if used in a position solution, can help produce much better position solutions. Without such real-time ionospheric delay measurements, mathematical models or measurements taken by third parties (which can be old) must be used instead. The communication of this information to a rover (also known as a mobile or remote unit) can be costly and require wide communications channel bandwidths.
Since selective availability was turned off in May 2000, commercial (i.e. non-military) users are now able to use differential GPS (DGPS) to make position determinations that are accurate to less than a centimeter. With DGPS, a stationary reference station is placed at a very accurately known location. The reference station generates corrections that are sent to rovers that are relatively close to the remote positioning receiver. A DGPS user receives these corrections and applies them to direct GPS measurements that it makes. This gives a user a position solution of very high accuracy.
However, as a remote positioning receiver is moved away from a reference station, the position solution accuracy will be reduced. For the accuracy required for survey applications, carrier-phase tracking receivers are generally required. The L1 and/or L2 carrier signals are used for carrier-phase tracking. Carrier-phase tracking involves measuring differences in carrier phase cycles and fractions of cycles. In DGPS systems, both a reference station and a rover must track the carrier signals at the same time. Atmospheric delay differences at the remote positioning receiver and the rover must be small enough to ensure that both receivers track the same carrier phase. Thus, for carrier-phase surveying (i.e., DGPS with sub-centimeter accuracy), the rover should be within approximately 30 Km of the reference station.
DGPS systems offer greatly improved accuracy over standard GPS systems, but there are some problems in implementing DGPS. For example, in some areas of Europe, UHF/VHF radio transmissions may be limited to 0.5 watts, which severely limits the range at which a rover can receive corrections. Further, direct broadcast of correction data limits a user""s ability to post-process data to verify that the remote positioning receiver was operating correctly when critical measurements were made.
A position determining system using a first communication network, a second communication network, and a positioning system is disclosed. The first communication network may be a packet-switched network, such as a frame relay network or an ATM network. The second communication network may include a public switched telephone network, a CDMA or other wireless network, or any communications network suitable for transmitting GPS data.
The system may also include at least one reference station that has accurately known position coordinates. The reference station can receive position signals from the positioning system, which may be a satellite-based system, such as the NAVSTAR GPS positioning system. Alternatively, the reference station can receive position signals from an earth-based positioning system or another satellite-based system, such as the GLONASS positioning system.
The reference station may be adapted to transmit position data via the first communication network in response to the reference position signals and the known position coordinates. The position coordinates of the reference station can be determined by any known survey technique, or, alternatively, the remote positioning receiver""s position may be calculated by using averaging techniques. In averaging, position signals received from the positioning system over a statistically significant period can be used to determine the reference station""s position.
A remote central host may be communicatively coupled to the reference station via the first communication network. The remote central host can receive and store the position data (e.g., raw GPS data and real-time kinematic data) transmitted from the reference station and further, the remote central host can generate and transmit correction data in response to the first position signals and the known position coordinates of the remote positioning receiver. The correction data may be sent via the first or second communication network to provide either real-time or archived position corrections to a rover.
The rover is a GPS receiver that is capable of receiving position signals from the positioning system. The rover may also be communicatively coupled to the first or second communication network so that it can receive and use the correction data and the second position signals (i.e., position signals not received at the remote positioning receiver) to accurately determine the rover""s position. The data sent by the remote central host could be either real-time or archived. The network may provide any-to-any connectivity for a roverxe2x80x94that is, the rover may receive correction data from virtually any communication network, wireless or land-based, circuit-switched or packet-switched, so that the cost and speed requirements of the user may be accommodated. For example, if the rover must have real-time data regardless of cost and the rover is mobile, the rover may communicate with the remote central host via a wireless communication network, such as a CDMA or other cellular network. If, on the other hand, the rover is in a static position, such as a structural monitoring site, it may be connected to the remote central host directly via landline modem, or it may even access correction data from the remote central host via the Internet.
Differential global positioning systems (DGPS) can greatly enhance the position solution of a mobile receiver known as a rover that is close to a reference station that has an accurately known position. To accomplish this enhancement, it is necessary to transmit position correction data, also known as differential data, from the reference station to the rover. Using the disclosed architecture allows for correction data to be sent to a remote central host in real-time, and retrieval of the correction data may mitigate many of the limitations of direct transmission of correction data. The correction data can be retrieved from the central host by a variety of communications media, such as (without limitation) a mobile telephone network, a circuit-switched network, a packet-switched network, or radio broadcast for use by a rover.
These and other features and advantages of the invention will be more completely described below in the detailed description of the exemplary and alternative embodiments of the invention.